[Update (18:00 GMT): Well,it looks like it really may have been an impact! Smaller black spots have been found, and the shape of the big spot indicates a trajectory for the incoming bolide. I’m still waiting for images and more info, but for now this is looking much more like an impact and not a simple weather event as I speculated.]

People on Twitter (and on that site linked above) are speculating that this is the result of a large impact on Jupiter, similar to what happened in 1994 when comet Shoemaker-Levy 9 slammed into the big planet. But I think that’s premature; weather changes rapidly on Jupiter, and black spots have appeared before — see here and here. That doesn’t mean this isn’t an impact event (or, for that matter, the earlier events weren’t either) but it’s jumping the gun to call it an impact event before we get more observations. I will add that this new spot is indeed very dark, which means it’s pretty interesting no matter what it is!

An irony: I am at LaunchPad, a workshop about astronomy for science fiction authors, and tonight we’re planning on visiting the Wyoming Infrared Observatory, where they have a 2.3 meter IR ‘scope, which is perfect to observe something like this. Even better, the spot should be visible tonight while we’re there! The irony is, dagnappit, that it’s very cloudy here for the first time in days. I’m hoping this cloud layer will blow out so we can get a chance to see this rare and interesting phenomenon on Jupiter.

Oh, and more irony? In a few minutes, as I write this, I will be giving my "Death from the Skies" talk, which focuses specifically on asteroid and comet impacts. Irony can be pretty ironic sometimes.